The invention relates to a method of utilizing secondary raw materials containing iron, zinc and lead, preferably steelmaking dusts, in a rotary tubular furnace customarily equipped for the rolling process, with basically adjusted rolling slag.
It is known that materials containing zinc and lead are mixed with coke breeze (carbon carrier) during the rolling process and are charged to a rotary tubular furnace operating on the counterflow principle. Due to the inclination of the furnace center line and the rotation of the furnace, the material moves to the deeper end of the furnace. The counter-flowing air, which is heated by a burner multiple times, oxidizes the gases resulting from the burdening, whereby the temperature of the gases rises and the furnace chamber and the burden are heated.
When travelling through the furnace, the burden undergoes a change of composition such that carbon reacts with reducible iron oxides first by forming metallic iron, that resulting carbon oxide and solid carbon then react with zinc oxide to form zinc metal, that lead and compounds thereof react correspondingly and that eventually everything vaporizes in correspondence with its vapor pressure ratios. The metallic iron is carburized by excess carbon. The solid residues called rolling slag contain excess coke, carburized metallic iron and a remainder of slag-forming oxides, and also small amounts of zinc. The flue gases lose the content of free oxygen on their way through the furnace. In an ideal rolling operation, the flue gases neither contain O2 nor CO at the gas outlet.
In the previously known basic processes the rolling slags are characterized by eluate values and building-physical properties, which limit the utilization thereof to a few fields of application, such as the construction of waste dumps.
Additional disadvantages of the previously known basically performed rolling processes are:
1. High energy consumption due to high coke growth.
2. Higher energy consumption due to frequent additional firing with a high-grade combustible.
3. Waste of energy due to 5-10% residual coke and metallic iron ( greater than 90% of the prerun) in the rolling slag.
4. Occurrence of ferriferous scaffolding at the furnace walls or of iron pellets in the proximity of the discharge area, favored by high carbon contents in the iron sponge.
For avoiding said disadvantages, suggestions are provided in the literature, namely to achieve an oxidation of the rolling slag by blowing up hot air at the slag outlet.
In the method according to U.S. Pat. No 3,66,522 hot air is blown up at the discharge area of a rotary tubular furnace, which is heated to 700xc2x0 C.-750xc2x0 C. in a separate recuperator unit, without claiming that the energy required therefor is taken from the process. The temperature in the top-blowing area is to be capable of rising up to 2000xc2x0 C., which also follows from the flow temperature of the reaction partners. The rolling slag will melt at least partially, it is to be in the form of granules. The method is preferably meant for pyritiferous ores.
Nearly identical with the principles of said method is the known rolling method according to EP 0 654 538. Here, too, hot air of 500-1000xc2x0 C. is blown into the discharge area of the rotary tubular furnace so as to include the oxidizable constituents of the rolling slag in the energy utilization. By burning excess carbon and metallic iron the temperature of the rolling slag rises up to 1200xc2x0 C.-1500xc2x0 C., it is, however, meant to remain crumbly and not to flow out in a melted form. Therefore, the method can only be used in connection with a basic operating mode. For limiting the thermal profile in the gas chamber of the furnace the air volume is so dimensioned, that the flue gas at the uptake of the furnace (charging side for the burden) still contains CO and Zn vapor beside other components, i.e. that it is substoichiometric in view of the combustion. Therefore, a controlled subsequent combustion is necessary. The hot air preparation in this method has been solved in a laborious manner. The waste gases of the subsequent combustion serve the preheating of the combustion air, for which a special recuperator is required, which operates under unfavorable conditions (dust, chlorides, lead). Technical solutions for such a recuperator are not known.
The invention is based on the object to provide a method of performing a rolling process with basically adjusted slag in normally equipped rolling plants with a justifiable technical expenditure. The method is to operate at a temperature in the rolling slag lower than the previously usual one and without a subsequent combustion of waste gas with air outside the rotary tubular furnace. Further, the energy balance of the rolling process is to be improved and the throughput of the used rotary tubular furnace is to be increased. Further, the quality of the slag is to be improved so that it becomes utilizable in a more versatile manner.
According to the invention these objects are solved by a method according to claim 1. Preferred embodiments are described in the dependent claims.
In the method according to the invention secondary raw materials containing iron, zinc and lead, preferably steelmaking dusts, are processed in a rotary tubular furnace operating on a counterflow principle in view of burden and gas atmosphere, with basically adjusted rolling slag such that the raw materials are mixed and/or agglomerated with a reactive fine-grained carbon carrier, with the quantitative portion of the carbon being strongly substoichiometric in contrast to all carbon-consuming reactions in the burden, that an additional portion of coarse-grained carbon carriers is distributed between the agglomerates, that the total portion of the carbon is  less than 80% of the quantity required by all carbon-consuming reactions in the burden at a rolling temperature of  less than 1150xc2x0 C. It is preferred that the carbon portion is so dimensioned that the rolling slag does not contain any free carbon ( less than 1%) even without a subsequent oxidation, that a total air volume is supplied to the furnace, which is stoichiometric or respectively hyperstoichiometric in view of all oxidizable gas constituents, i.e. it can do without a special controlled subsequent combustion, that the rolling slag is supplied with cold air in the proximity of the furnace outlet upon reaching a stable run of the furnace in an amount reducing the portion of metallic iron to less than 20%, preferably  less than 10%, and that a high degree of basicity of the rolling slag is adjusted with a high portion of magnesium with MgOxe2x89xa70.1%Cao also by adding lime and magnesium containing wastes, such as plaster precipitation sludge.
According to the invention the fine-grained carbon carriers have a diameter of about 0-6 mm, preferably 0-4 mm and most preferred  less than 2 mm. According to the invention the coarse-grained carbon carriers have a diameter of about 0-16 mm, preferably 0-12 mm and most preferred  less than 10 mm.
The method has the advantage that it is completely sufficient to cover an additional need of heat at the outlet of the furnace by deliberately introducing cold air in an amount of 30-40% of the total air volume. The air causes the oxidation of the metallic iron and the generation of heat related therewith. The temperature of the rolling slag can be adjusted to a temperature of  less than 1150xc2x0 C. by controlling the air volume. The absorbed air volume is controlled by the flue in a known manner. It is to be dimensioned in a way that the flue gas at the uptake of the furnace contains 0.5-2% O2.
In accordance therewith a furnace regime is achieved, in which the quantitative portion of the carbon in the layer of solid matter, i.e. in the furnace burden, is substoichiometric in view of the need of carbon for all carbon-consuming reactions, but allows a total combustion of all constituents in the gas chamber, thereby being stoichiometric or hyperstoichiometric, respectively, in view of the combustion with free O2 in the flue gas.
This principle is rendered advantageous by using a reactive and fine-grained carbon carrier, which also contains small amounts of volatile substances, which is charged to the furnace after having been mixed and/or agglomerated with the fine-grained raw materials and admixtures.
Said principle is further rendered advantageous by being capable of also using waste plasters as basic admixtures allowing a largely troublefree run of the furnace. The latter is additionally improved by the low carbon content of the formed metallic iron and its higher melting point effected thereby.
In accordance with the described operating mode, said principle allows a normal operation also of smaller furnaces largely without additional heating.
The technical realization thereof is effected by carefully dosing and mixing the components and by using an agglomerating plant. Furthermore, an air supplying device is required comprising a fan, measuring and controlling systems and a movable blowing lance. The resulting rolling slags can be used in the field of waste dump construction or in asphalt sub-bases. They are densely sintered in view of the surface, free of carbon ( less than 1%) and only contain small amounts of metallic iron. Their eluate values comply with the legal provisions.
The crumbly rolling slag is granulated or quenched in a water beam upon its discharge from the furnace.
In the method according to the invention the energy balance of the rotary tubular furnace and also the throughput of the utilized rotary tubular furnace are improved by the reduction of the amount of carbon carriers and by the utilization of the energy potential of the metallic iron at the furnace outlet.